Interstellar Planet Formation

Continuing the discussion from last month’s blog about planetessimal-building conditions in space beyond the solar system’s heliopause boundary (1). In my February paper, I discussed anomalous results which had come back from various space probes regarding the influx of large grain interstellar dust into the heliosphere (2). More on this in a moment. A correspondent of mine had noted similarities between what I had been writing about and previous work by Paul LaViolette, who had written about the origins of the dust picked up by the Ulysses spacecraft:

“I would suggest that the dust originates from a circumsolar dust sheath that is concentrated toward the plane of the ecliptic in a fashion similar to the disk girdling the star Beta Pictoris and that is co-moving with the Sun. Infrared observations confirm the existence of dust sheaths around other stars in the solar neighborhood, leading to the conclusion that our Solar System is similarly shrouded.” (3)

The 20 million year old star Beta Pictoris provides astronomers with the best example of a gas giant exoplanet found orbiting within an evolving proto-planetary disk, made all the more dramatic by its side-on view and the brightness of scattered light from the revolving disk:

“In 1984 Beta Pictoris was the very first star discovered to host a bright disc of light-scattering circumstellar dust and debris. Ever since then Beta Pictoris has been an object of intensive scrutiny with Hubble and with ground-based telescopes. Hubble spectroscopic observations in 1991 found evidence for extrasolar comets frequently falling into the star.” (4)

Such a theme is also explored, at last from the point of view of how the solar system might have appeared early in its history, by astrophysicist Dave Jewitt in his highly informative online article about the Edgeworth-Kuiper Belt:

“One suggestion is that the depletion of the Kuiper belt could have been caused by extensive collisional grinding of the objects into dust. The dust would have temporarily given the Solar system a ring-like appearance as observed in the debris disks of nearby stars (Moro-Martin et al. 2007, Wyatt 2008) but, over time, the dust would have been swept away by radiation forces. Alternatively, some suppose that the Kuiper belt was dynamically depleted, presumably by interactions with Neptune.” (5)

{The flux of dust grains impacting spacecraft in the outer solar system (5). The impact flux corresponds to dust production rate of two tonnes per second. From Vitense et al. (2012) (6)}

Dr Jewitt describes how there is an ‘armada’ of in-falling dust and other debris from the Kuiper Belt created by the continuous risk of collisions among the literally trillions of objects which make up the Kuiper Belt. This interplanetary dust has been detected by various spacecraft, and extends deeply back through the planetary zone of the solar system. That said, the Kuiper Belt lies well within the heliosphere. It is what is going on beyond which is of keen interest. My contention, as well as that of Paul LaViolette before me, is that beyond the heliopause such depletion by radiation forces does not take place, leaving an extant sheath of dust. I am arguing, further, that this sheath is gradually swept up by planetary bodies lying beyond the heliopause to create dust disks around them, and that this sheath of interstellar dust is periodically rejuvenated when the solar system moves through areas of relatively dense dust clouds interspersed throughout the galaxy.

You might reasonably ask how such a sheath might have avoided direct detection? In this context, it’s interesting to note that the Kuiper Belt is also thought to have a debris cloud, but one that has not been directly detected because of the ‘zodiacal light’ reflected back at us from interplanetary dust in the foreground:

“In contrast to all other debris disks, where the dust can be seen via an infrared excess over the stellar photosphere, the dust emission of the Edgeworth-Kuiper belt (EKB) eludes remote detection because of the strong foreground emission of the zodiacal cloud.” (6)

If that’s the case for a debris dust disk thought to exist in the Kuiper Belt, then it most assuredly applies to any extensive dust disk lying beyond the heliosphere, which is located about twice the distance away. Simply put, if such a sheath of dust exists beyond the heliopause, then the comparatively bright foreground light of the interplanetary dust in the inner solar system is effectively preventing its detection. Only a spacecraft moving beyond the heliopause with dust-detecting equipment will know for sure.

The only probes exploring beyond the heliopause at the moment are the Voyager spacecraft, with the Pioneers somewhat behind them. The Voyager spacecraft are not equipped with actual dust detectors, but will record the electric effects of the plasma cloud generated during the high velocity impact between dust and spacecraft, and even potentially pick up evidence of charged particles passing between their electric antennae. However, interpreting this data is not necessarily very straightforward:

“On November 13,1980, the spacecraft Voyager 1 passed through Saturn’s E ring – a faint dusty structure having a rather large radial extension and thickness. The Voyager spacecraft did not carry conventional dust detectors, but they carried two wave instruments designed for measuring radio and plasma waves. Both instruments recorded dust signals, but this was recognised much later because the plasma played a dirty trick: genuine dust signals were mixed with intense plasma waves, and that made them difficult to interpret.” (9)

As interstellar medium, including dust, likely takes the form of diffuse plasma, then these spacecraft are still capable of sending back data about what they’re encountering as they shoot through the heliosheath zone. There are complications, though. Larger particles of particulate matter will be mixed in with what is presumed to be a steady flow of interstellar plasma. Furthermore, the rather archaic data return from the Voyager craft these days is necessarily limited by the reducing power output of their slowly dying batteries as their nuclear fuels are depleted. Nonetheless, scientists were still able to ascertain that Voyager entered an unexpected, but immense cloud of interstellar plasma held together by a strong magnetic field (10):

“Astronomers call the cloud we’re running into now the Local Interstellar Cloud or “Local Fluff” for short. It’s about 30 light years wide and contains a wispy mixture of hydrogen and helium atoms at a temperature of 6000 C. The existential mystery of the Fluff has to do with its surroundings. About 10 million years ago, a cluster of supernovas exploded nearby, creating a giant bubble of million-degree gas. The Fluff is completely surrounded by this high-pressure supernova exhaust and should be crushed or dispersed by it.

“The observed temperature and density of the local cloud do not provide enough pressure to resist the ‘crushing action’ of the hot gas around it,” says [Merav]Opher [a NASA Heliophysics Guest Investigator from George Mason University]. So how does the Fluff survive? The Voyagers have found an answer. “Voyager data show that the Fluff is much more strongly magnetized than anyone had previously suspected—between 4 and 5 microgauss,” says Opher. “This magnetic field can provide the extra pressure required to resist destruction.” (11)

Perhaps the issue here is more to do with the perceived density of this cloud, at least at a very local level. The zones beyond the heliosphere may be awash with interstellar clouds; even flowing rivers of interstellar plasma. But what about denser, heavier materials? What about larger grained material which is not ionised into plasmas?

An anomalously high level of large dust grains has been observed by the Ulysses, Cassini and Galileo satellites, which moves with the ISM flow towards the inner solar system. This effect has yet to be satisfactorily explained, and implies that there is an unexpected source for these particles beyond the heliosphere (12). Calculations, based upon certain assumptions, indicate a possible source ranged some 500 AU away (13), well beyond the heliosheath zone.

Astrophysicists interested in debris disks in the outer solar system have done modelling on whether dust can clump together at the distance of the Kuiper Belt. They have found from their calculations that dust which falls into a resonant orbit with Neptune can indeed clump, forming larger particles of debris matter which can statistically withstand the opposing mechanisms of collisional cascades (14). (Such collisional cascades are thought to account for an observed dust cloud surrounding a presumed giant exoplanet lying in the extensive debris disk orbiting the star Formalhaut (15)).

In other words, dust in the Kuiper Belt can, under certain conditions, begin to accrete – at least hypothetically, anyway. Such a mechanism has not been detected directly, for the same reasons described above – interplanetary Zodiacal dust blinds us to more subtle effects beyond, in the same way street lighting impedes stargazing. What’s important here is the shepherding influence of a major nearby planet which, in the case of the Kuiper Belt, is Neptune.

So, one would assume that a similar catalyst would be required beyond the heliopause. One requires the influence, then, of a Planet Nine (16), Planet X or Dark Star object. Such a mechanism may explain the anomalous influx of materials from beyond the heliopause, and provide an on-going process of planet-building in interstellar space.